1. Field of the Invention
This invention relates to the conversion of isobutyric acid or its equivalents and lower alkyl esters thereof correspondingly to methacrylic acid or its equivalents and lower alkyl esters thereof.
2. Description of the Prior Art
There exists considerable prior art relating to the oxydehydrogenation of the lower saturated aliphatic monocarboxylic acids to produce the corresponding alpha, beta-olefinically unsaturated acids. Early work in this area involved thermal, vapor phase oxydehydrogenation of the saturated aliphatic carboxylic acid in the presence of oxygen and iodine. This approach has not been particularly successful from a commercial standpoint. This is understandably so inasmuch as iodine is costly, exhibits extreme corrosion properties and poses considerable problems in realizing complete recovery of the comparatively large amounts thereof required in the process. The heterogeneous catalytic method of oxydehydrogenation according to the prior art appears to be the more attractive route to the commercial production of alpha, beta-olefinically unsaturated monocarboxylic acids. The prior art heterogeneous oxydehydrogenation catalysts useful for this purpose include some heteropoly acids, such as phosphomolybdic acid, optionally with tungsten and/or vanadium (U.S. Pat. No. 4,061,673). Another type of prior art catalyst useful in the oxydehydrogenation of aliphatic carboxylic acids and esters thereof is iron phosphate.
Iron phosphate subjected to calcination exists in several crystalline phases or species. It is not known at this time which species is or are catalytically active. There is evidence that the presence of certain extrinsic metal components in the catalyst preparation serves to facilitate the formation of an active catalyst. U.S. Pat. No. 3,948,959, for instance, discloses that an alkali or alkaline earth metal can be the extrinsic metal for this purpose. In general, oxydehydrogenation catalysts of the type discussed above function best when relatively large quantities of steam are included in the feed along with the carboxylic acid and molecular oxygen. The prior art catalysts also usually work best at relatively higher temperatures. In contrast, the catalysts embodied in the process of my invention work best at relatively lower reaction temperatures with relatively lower amounts of steam in the reactor feed.